This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Cell proliferation depends on intracellular and extracellular polyvalent interactions. The development of novel approaches to treat diseases requires the development of effectors and inhibitors that exploit the polyvalent basis of numerous processes in cell proliferation. The development of effective polyvalent ligands for proteins, DNA, and cell surfaces requires a multidisciplinary approach that includes the combination of (a) optimized ligands for the individual binding sites;(b) spacer elements capable of highly precisely separating the monovalent ligands for optimized interaction at multiple sites on the target to maximize favorable enthalpy and entropy without introducing the significant entropic or enthalpic penalties commonly observed in polyvalency;and (c) ligation methodologies to link the multiple ligand and spacer elements in a precise manner. In this subproject, we describe a multidisciplinary approach to develop novel peptide-based antagonists of the proliferating cell nuclear antigen (PCNA). PCNA is a trimeric protein that is a master regulator of cell proliferation via its interactions with numerous proteins including the replicative DNA polymerase. The tumor suppressor p21 functions by binding to PCNA and preventing other cellular proteins from accessing PCNA. We are developing approaches to achieve highly effective ligands of PCNA via a multidisciplinary approach that employs the following specific aims: (1) The development of novel PCNA ligands by peptide phage display and from cyclic peptide libraries;(2) the development of assays of PCNA binding and antagonism of DNA polymerase activity;(3) the development of bivalent and trivalent inhibitors of PCNA via the development of novel spacers and linkers capable of optimally placing multiple ligands on PCNA and preventing PCNA interaction with other cellular targets. This approach, in which individual ligand, spacer, and linker elements are rapidly combined to identify optimized ligands, is being used as a model system for the development of new classes of inhibitiors, toward future application as a general approach to control cell communication and cell proliferation. Broadly considered, this work will develop new approaches to understand and control communication between cells and within cells, which is important in inflammation, cancer, bacterial infection, and tissue regeneration. These approaches will have applications to understand the causes of disease and in the development of novel and effective treatments for disease.